230 ROMANCE OF THE INSECT WORLD CHAP. 



may depend upon it that advantages to the species 

 thereby do arise, else one of the forms would be 

 eliminated. Probably a dimorphic* species is better 

 protected than one which is non-dimorphic. The 

 larvae of the former have a wider range of resem- 

 blances to the tints and objects of nature, and there- 

 fore more chances of concealment and escape from 

 enemies. Dimorphism would also seem to bestow 

 greater adaptability on the part of a species to new 

 scenes. In its wanderings one of its forms may be 

 well protected in the new country ; in other countries 

 the other form may be more concealed. The form 

 best in harmony with the new situation would tend to 

 develop, to the subduing and exclusion of the other 

 a command of resource in which a non-dimorphic 

 species would be far inferior. Natural selection 

 would appear to be the power at work, effecting 

 the disappearance of the less protected variety. 



Among perfect insects dimorphism seems to in- 

 dicate the development of a younger from an older 

 form. One form is usually much rarer than the 

 other, and probably the older of the two. The later 

 development may be ascribed in part to its increased 

 power of protection of the species. Although in most 

 cases the two or three forms are now distinct, it is 

 probable that they were formerly connected by 

 intermediate gradations. Thus Dr. Wallace describes 

 a butterfly in the Malay Archipelago which presents 

 a great number of connected varieties, and the extreme 

 links of the chain closely resemble the two forms of 

 an allied dimorphic species inhabiting the same 

 country. Darwin also points out that though with 



